A digital subscriber line (DSL) is a high-speed data transmission technology using a phone twisted pair for transmission. G.fast is the latest DSL technology, and is also referred to as a gigabit DSL. The technology uses a high frequency band of 100 megahertz (MHz) or even 200 MHz. In a high frequency band, a very serious crosstalk problem exists and needs to be resolved. Crosstalk includes near-end crosstalk (NEXT) and far-end crosstalk (FEXT).
G.fast uses a time division duplexing (TDD) technology such that upstream and downstream signals are sent at different time points, to eliminate an effect of NEXT, and uses a vectoring technology to eliminate an effect of FEXT.
A vectoring system includes a central office (CO) end (or a distribution point unit (DPU) end, where the CO end and the DPU end are subsequently collectively referred to as a CO end) and a far end. The CO end includes multiple (M) transceivers. The far end includes multiple (M) corresponding transceivers. During downstream transmission, the M transceivers of the central office end are used as transmit ends, the M transceivers of the far end are used as receive ends, and each transmit end corresponds to one receive end.
In a downstream direction, signals sent by transceivers of each CO end pass through a precoder before an inverse fast Fourier transform (IFFT) module. Most DSLs use a discrete multitone (DMT) modulation technology, and use a maximum of K subcarriers. Therefore, a signal of each office-end transceiver (the mth office-end transceiver) that has not passed through the precoder is xm, which is a sequence including K subcarriers. A signal that is sent by the mth (m=1 to M) transceiver on the kth (k=1 to K) subcarrier and that has not passed through the precoder is marked as xkm, and a signal that is sent by the mth transceiver on the kth subcarrier and that has passed through the precoder is marked as xM′k. Signals that are sent by all the M transceivers on the kth subcarrier and that have not passed through the precoder are marked as xk, and signals that are sent by all the M transceivers on the kth subcarrier and that have passed through the precoder are marked as x′k, where xk and x′k are both vectors having M rows and one column, and the mth element of xk and the mth element of x′k are respectively xmk and xm′k.
Therefore, on the kth subcarrier, a downstream signal transmission process may be expressed in a form of the following matrix: x′k=Pkxk. On the kth subcarrier, a received-signal vector yk of M twisted pairs may be expressed in a form of the following matrix: yk=Feqk·(Hk·Pk·xk+γk)yk=Feqk·(Hk·Pk·xk+γk). When Feqk·Hk·Pk is equal to an M*M unit matrix IM, all crosstalk is canceled (a received signal of each line is only related to a signal of the line, and is unrelated to another signal), where Pk is a precoding matrix on the kth subcarrier, and has a size of M*M, a module implementing a function of the precoding matrix is the precoder, and the module is also a module implementing a function of the vectoring system. Hk is a channel matrix on the M twisted pairs on the kth subcarrier, and a size of the matrix is M*M. Feqk is a frequency domain equalizer (FEQ) matrix on the M twisted pairs on the kth subcarrier, the matrix has a size of M*M and is a diagonal matrix, a function of an FEQ is to restore a received signal to a sent signal, and a downstream FEQ is implemented in a transceiver xTU-R of the receive end, and a theoretical value of Feqk is a reciprocal of a diagonal of Hk, and γk is a noise vector of M receivers on the kth subcarrier, and a size of the vector is M*1.
In an upstream direction, after a received-signal vector Y is processed using a cancellation matrix, a received signal Y′ that has passed through the cancellation is generated. A formula may be expressed asy′k=Feqk·Wk·(Hk·xk+γk),where Wk is an upstream crosstalk cancellation matrix.
A method for acquiring the precoding matrix in an existing system is as follows. A pilot training signal is sent on a sync symbol, and a vectoring control entity (VCE) estimates the channel matrix Hk or a normalized channel matrix Heqk (the normalized channel matrix Heqk equal to Feqk·Hk, and is a matrix whose diagonals are all equal to 1) using the pilot. Hk and Heqk are collectively referred to as a channel matrix. A signal that is sent by a transceiver unit (TU) at the transmit end on the sync symbol is usually an orthogonal pilot sequence (PS) signal. Meanwhile, a TU at the receive end receives an error sample (ES). The channel matrix Heqk may be estimated using the ES and the PS. After the channel matrix Heqk is obtained through training, a cancellation matrix P or W, which is Heqk−1, is obtained using a method of inversion or approximate inversion. A module calculating the cancellation matrix is the VCE.
A description about a data symbol and a synchronization symbol of G.fast is as follows.
A data transmission unit of G. fast includes a symbol, a TDD frame and a superframe. In G. fast, sending is performed in an upstream-downstream time division manner. Each TDD frame has a length of 36 symbols, among which 35 data symbols are used to send upstream and downstream data in a time division manner. Therefore, there are a total of 35 upstream and downstream symbols. In each TDD frame, a former part is used to send downstream symbols, and a latter part is used to send upstream symbols. The other symbol is a gap between upstream sending and downstream sending, and is not used for sending a signal. Eight TDD frames form one superframe. Therefore, one superframe has a length of 288 symbols, among which there are 280 signal symbols. Among the 280 signal symbols, there is one upstream sync symbol and one downstream sync symbol, onto which only a PS or an online reconfiguration (OLR) control signal is modulated, and the other 278 signal symbols are data symbols, and are used to transmit user data.
Considering environmental protection and an energy consumption requirement of remote power supply, power consumption of a transceiver needs to be reduced as much as possible. Currently, in the DSL field, there is a discontinue mode for power saving (DMPS) technology. The main solution of the technology is that when there is no service data, a digital front end (DFE) processing module, an analog front end (AFE) processing module, and the like are turned off. The DFE processing module is, for example, a fast Fourier transform (FFT) module or an IFFT module. The AFE processing module is, for example, a digital-analog conversion (ADC/DAC) module or a line driver module. Making these modules stop working can greatly reduce energy consumption of a system.
Turning off analog front ends of some lines is equivalent to setting output signals of downstream precoding of the lines to zeros or setting an input signal of an upstream canceller to a zero. The setting to zeros has serious effects on a vector cancellation effect. Examples are as follows.
During normal transmission, an entire channel including a precoder is a product Feqk·Hk·Pk of three matrixes. The product is a unit matrix. Therefore, the crosstalk is completely canceled.
If analog end signals of some lines are suddenly turned off, it is equivalent to setting rows corresponding to the turned off lines in Pk to zeros. Assuming that a set of index numbers of the turned off lines is deactivated (d), a set of index numbers of lines that are not turned off is active (a), a channel after the turn off is,
                    Feq        k            ·              H        k            ·              [                                            0                                                                          P                                  d                  *                                k                                                    ]              =                                        Feq            k                    ·                      H            k                    ·                      P            k                          -                              Feq            k                    ·                      H            k                    ·                      [                                                                                P                                          d                      *                                        k                                                                                                0                                                      ]                              =              I        -                              Feq            k                    ·                      H            k                    ·                      [                                                                                P                                          d                      *                                        k                                                                                                0                                                      ]                                ,where Pka is a row that is in the precoding matrix and that corresponds to an active line, and Pkd is a row that is in the precoding matrix and that corresponds to a turned off line.
After the lines of the d set are turned off, an error (a difference relative to a unit matrix I) generated on final lines is
      Feq    k    ·      H    k    ·            [                                                  P                              d                *                            k                                                            0                              ]        .  The error is not a diagonal matrix, and may generate relatively serious residual crosstalk and affect performance of all active lines.
One method for resolving the problem of residual crosstalk is to calculate an inverse matrix of a submatrix of an active line a according to an index of the active line. Specific implementation is as follows.
Assuming that an inverse matrix P of an entire channel matrix H is represented
            H              -        1              =          P      =              [                                                            P                aa                                                                    P                ad                                                                                        P                da                                                                    P                dd                                                    ]              ,as an inverse of an a-index submatrix of the H may be represented as Haa−1=Paa−PadPdd−1Pda.
The precise calculation method requires inversion performed on a submatrix Pdd. A symbol rate of G.fast is 48 kilohertz (kHz), and each symbol only has 21 microseconds. Therefore, it is almost impossible to complete the inversion calculation in 21 microseconds.
Further, to avoid the inversion calculation, there are some similar signal compensation methods. An example is as follows.
A precise compensation theoretical formula is as follows, but the precise compensation has an inversion complexity problem,X′a=Haa−1Xa=PaaXa−PadPdd−1PdaXa.
To avoid the problem, approximation may be performed on an inversion operation of Pdd,Pdd−1≈Ddd−1(I−BddDdd−1),where Ddd is a diagonal matrix that includes only a diagonal element of Pdd, and Bdd is a matrix that includes only a non-diagonal element of Pdd and whose diagonal is 0. Ddd is a diagonal matrix. Therefore, inversion is merely calculating a reciprocal of the diagonal element, and requires a very small amount of calculation, and the calculation can be completed in advance. Therefore, approximate compensation using the following formula does not require a large amount of calculation, and can be basically completed.X′a=PaaXa−PadDdd−1(I−BddDdd−1)PdaXa.
In a low-power sending state, it is required that a transceiver is turned off whenever possible in order to save power. Therefore, if a port needs to be deactivated, an analog front end of the transceiver also needs to be turned off as soon as possible.
In an existing Very-high-bit-rate digital subscriber line 2 (VDSL2) vectoring system, during deactivation, a vectoring coefficient does not need to be updated.
In a deactivation process in which a line changes from an active state L0/L2 to an L3 state, transceivers of two sides perform state switching after negotiation.
But the existing deactivation process is only applicable to a weak crosstalk scenario, that is, a scenario in which a channel matrix H is strictly diagonally dominant (a non-diagonal element of H is far less than a diagonal element). It is assumed that there are a total of m lines in a current vectoring system, among which a lines remain active and d lines are deactivated.
Therefore, before deactivation, signals of the a lines are sent using the d lines. A formula is expressed as
      X    ′    =            (                                                  X              a              ′                                                                          X              d              ′                                          )        =                  P        ·        X            =                                    [                                                                                P                    aa                                                                                        P                    ad                                                                                                                    P                    da                                                                                        P                    dd                                                                        ]                    ⁢                      (                                                                                X                    a                                                                                                                    X                    d                                                                        )                          =                              (                                                                                                      P                      aa                                        ·                                          X                      a                                                                                                                                  P                      ad                                        ·                                          X                      d                                                                                                                                                              P                      da                                        ·                                          X                      a                                                                                                                                  P                      dd                                        ·                                          X                      d                                                                                            )                    .                    
After deactivation, analog front ends of the d lines are turned off, and precoded signals Xd′ of the d lines cannot be sent out. Therefore, some signals cannot be sent out (signals Pda·Xa refer to signals of the a lines that pass through a precoder P and are sent by d). Because the signal Pda·Xa is not sent out, crosstalk between the a lines is not completely canceled, and therefore, a performance of the a lines is decreased.
In a VDSL2 scenario, crosstalk is relatively weak, and H is strongly diagonally dominant. Therefore, P is also strongly diagonally dominant. An amplitude of Pda is so small that the errors can be ignored. In a super high frequency DSL, crosstalk is very strong, the amplitude of Pda is obviously increased, and the errors cannot be ignored, which causes serious performance degradation. Similarly, turning off analog ends of d may result in turning off of upstream signals and cause residual crosstalk between lines.